Conventionally, frames serving as the frameworks of image forming apparatuses such as copiers, printers, facsimile machines, or digital multifunctional machines have been metal frame structures assembled by fastening or welding metal pressed components or metal-based steel products together.
With the recent trend toward lower costs for image forming apparatuses, conventional metal frames are not sufficiently profitable. Moreover, with the trend toward smaller and lighter apparatuses, simplification of the structure, reduction in the number of components, and the like have been pursued, but the metal frame structures impose a limit on the reduction in size and weight.
In view of the reduction in cost, size, and weight for the frame structure, it is conceivable to employ a resin frame composed of a resin. However, in this case, there is a problem with regard to the strength.
On the other hand, in an image forming apparatus including image forming units (processing units) such as a photosensitive member's unit, a charging unit, and a development unit, a guide portion extending in an insertion direction of the image forming units may be provided in order to guide the image forming units during insertion into and removal from a main body of the image forming apparatus.
For example, JP 2001-281952A discloses that a unit mounted portion has rail portions serving as guide portions during insertion of a drum set on a drum set attachment/detachment side and rail portions serving as guide portions during insertion of a toner set on a toner set attachment/detachment side.
In the case where a resin frame is molded using a mold, in order to mold a guide portion that is formed so as to extend in the insertion direction of an image forming unit, when a mold opening direction of a mold for molding the guide portion is set to a direction parallel to the insertion direction of the image forming unit, problems as described below may arise.
That is to say, in the case of forming the guide portion so as to extend in the insertion direction of the image forming unit, the distance over which the mold slides with respect to the guide portion increases, leaving problems in producing the mold in terms of the structure of the mold, the cost of the mold, and the like.
Moreover, in the case of setting the mold opening direction of the mold to the direction parallel to the insertion direction of the image forming unit, there may be problems with the position accuracy or the ease of operation of the image forming unit because of the draft of the mold.
FIG. 10 is a perspective view showing a pair of rail portions (guide portions) G1, G2 formed by setting the mold opening direction of the mold to a direction parallel to the insertion direction in which the image forming unit is inserted.
As shown in FIG. 10, the draft angle θ of the mold is set to, for example, about 3° because a certain amount of slide travel for molding a guide portion during removal of the mold is required. In that case, a distance d1 between upstream end portions of the pair of guide portions G1, G2 in an insertion direction Y1 of an image forming unit F is larger than a distance d2 between downstream end portions of the pair of guide portions G1, G2, and so a gap Δd is created at the upstream end portions of the pair of guide portions G1, G2 between the pair of guide portions G1, G2 and the image forming unit F, resulting in a decrease in the position accuracy of the image forming unit F. This becomes more conspicuous as the amount of slide travel for molding the guide portion increases.
In addition, once the gap Δd is created at the upstream end portions of the pair of guide portions G1, G2 between the pair of guide portions G1, G 2 and the image forming unit F, during insertion of the image forming unit F, the image forming unit F tends to skew with respect to the pair of guide portions G1, G2, and thus a sliding portion F1 tends to get caught by the pair of guide portions G1, G2 in the course of insertion of the image forming unit F.